1. Technical Field
The present invention relates in general to storage devices within digital systems, and in particular to an improved edge-triggered latch that combines the data and clock generation path utilizing pass-transistor logic.
2. Description of the Related Art
Digital circuits, such as microprocessors and memory devices, typically use flip-flops as temporary storage devices. The most basic type of flip-flops operate with signal levels and are often referred to as latches. A latch with clock pulses as its control input is essentially a flip-flop that is triggered every time the clock pulse goes to a one or zero logic level. For ease of reference, it will be assumed that, as utilized herein, xe2x80x9clatchxe2x80x9d incorporates flip-devices and all clock-controlled latches.
Several categories of latches are known in the art including level-sensitive, master-slave, and edge-triggered. The present invention is directed to edge-triggered latch design. With reference to FIG. 1, there is depicted a conventional D-latch 100. In accordance with well-known D-latch design standards, D-latch 100 has two inputs, D (data) at a data input node 106, and C (control) at a clock input node 108. D-latch 100 generates a differential output at an output node 114 and a complementary output node 112. The outputs at nodes 112 and 114 cannot change state while the clock input at node 108 is at a logic 0 regardless of the value of D at data input node 106. A graphical representation of the input and output signals to and from D-latch 100 is provided in FIG. 2.
D-latch 100 is a positive edge-triggered latch, meaning that it triggers a data input at input node 106 only during a positive transition of a clock signal C. Two such positive clock signal transitions, 212 and 214, are depicted in FIG. 2. Pulse generating circuitry including Complementary Metal Oxide Semiconductor (CMOS) P-type Field Effect Transistors (PFETs) P2 and P3, and N-type Field Effect Transistors (NFETs) N3, N5, N4, and N6, is utilized to enable edge-triggered data propagation within D-latch 100 as follows.
Three CMOS inverters, I1, I2, and I3 produce a delayed complementary version of control signal C, illustrated in FIG. 2 as {overscore (C)}dd. Control signal C is logically ANDed with its delayed complementary counterpart {overscore (C)}dd by series NFET pairs N3 and N5, and N4 and N6, to produce a data evaluation window having a width 210. The resulting data evaluation window is felt at a pair of pull-down nodes 118 and 120 at the sources of NFETs N1 and N2, respectively.
During a data evaluation window at pull-down nodes 118 and 120 data propagates through D-latch 100. At the onset of a data evaluation window (i.e., at a rising edge of C), the biasing at pull-down nodes 118 and 120 enables a pair of CMOS inverters comprising P1 and N1, and P4 and N2, respectively, to pass the input data through an internal data path node 104 and into a storage node 102. From storage node 102, the data propagates through inverter I6 to complementary output node 112 and through inverters I4 and I7 to output node 114. PFETs P2 and P3 suppress noise by maintaining a logic high at internal data path node 104 during non-data transfer cycles.
An ideal D-latch design provides a temporally symmetric complementary output as well as an optimized balance between performance and electrical efficiency. However, as illustrated in FIG. 2, the data path for Q is longer (four logic stages) than that for {overscore (Q)} (three logic stages), resulting in an unbalanced differential output. In addition to having an unbalanced output, D-latch 100 suffers a reduced power efficiency due to periodic xe2x80x9cglitchesxe2x80x9d experienced by internal data path node 104 wherein a high-to-low transition occurs for each clock cycle even in the absence of a change in the data input. Finally, the performance of D-latch 100 in terms of transfer speed is reduced by its CMOS edge-triggering design wherein additional clock pulse generating elements N3, N5, N4, and N6 are required to enable edge-triggered data propagation through the CMOS inverters in the data path.
From the foregoing, it can be appreciated that a need exists for an improved clock-triggered latch wherein the foregoing limitations of conventional latches are minimized.
An edge-triggered latch that incorporates pass-transistor logic (PTL) in the data and clock generation paths is disclosed herein. In accordance with one embodiment, an edge-triggered latch includes a data input and at least one data path PTL transistor that passes data from the data input into a storage node in response to a latch trigger signal. A latch trigger circuit generates the latch-trigger signal in response to a clock signal transition.
All objects, features, and advantages of the present invention will become apparent in the following detailed written description.